Behavior Of Shiga Toxigenic Escherichia coli Relevant to Lettuce Washing Processes and Consideration Of Factors for Evaluating Washing Process Surrogates

Abstract

Postharvest processes for fresh produce commonly include washing in water containing antimicrobial chemicals, such as chlorine; however, if the antimicrobials are not present in sufficient levels, washing can promote the spread of contamination that might be present. To understand cross-contamination risk during washing, we tested a collection of Shiga toxigenic Escherichia coli (STEC), including O157:H7 and other non-O157 strains, for certain traits during washing of fresh-cut lettuce, i.e., sensitivity to sublethal chlorine levels and ability to cross-contaminate (detach from and attach to) lettuce in the presence of sublethal chlorine levels. Nonpathogenic E. coli Nissle 1917 (EcN) and Pediococcus pentosaceus lactic acid bacterial species (LAB) were included as potential washing process validation surrogates. As measured by extension of the lag phase of growth in media containing 0.15 ppm of chlorine, chlorine sensitivity varied among the STECs. Cross-contamination was assessed by evaluating transfer of bacteria from inoculated to uninoculated leaves during washing. Without chlorine, similar transfer to wash water and uninoculated leaves was shown. In 1 ppm of chlorine, cross-contamination was not detected with most strains, except for the substantial transfer by a STEC O111 strain and EcN in some replicates. Strain O111 and EcN showed less inactivation in 0.25 ppm of chlorine water compared with O157 (P < 0.05). LAB showed similar transfer and similar chlorine inactivation to O157. Considering together the sublethal chlorine sensitivity and detachment/attachment traits, neither EcN nor LAB displayed optimal characteristics as washing process surrogates for the STEC strains, although further evaluation is needed. This work demonstrated a range of behaviors of STEC strains during lettuce washing and may be helpful in hazard characterization, identifying factors to consider for evaluating washing process efficacy, and identifying phenotypic traits to select surrogates to validate washing processes.

Pre-Harvest Survival and Post-Harvest Chlorine Tolerance of Enterohemorrhagic Escherichia coli on Lettuce

“…In our study, we also identified variability in chlorine tolerance among strains of EHEC. Differences in sanitizer tolerance among serotypes and/or genetic subtypes of foodborne pathogens have been reported for EHEC [38,39]. Shen et al examined variation in chlorine tolerance in produce wash water among multiple serotypes of Salmonella and Shiga toxin-producing E. coli (STEC), and found, consistent with our day 0 findings, that among STEC, O157:H7 strains were more tolerant than the non-O157 STEC tested [39].…”

Guidelines To Validate Control of Cross-Contamination during Washing of Fresh-Cut Leafy Vegetables

“…Transfer from contaminated leaves to wash water (58) and attachment to cleaned leaves (24) have been studied as relevant behaviors in the survival of E. coli O157:H7 in fresh produce processes. Characteristics for surrogate behavior that might be relevant in a cross-contamination event include inactivation by the antimicrobial agent, as well as ability to detach from and attach to leaves in a washing process (14). When such a surrogate has been identified, an option 1 validation could be performed by product inoculation, representing the worst-case pathogen contamination load reasonably likely to be introduced to the process.…”

Microbiological hazards associated with the use of water in the post‐harvest handling and processing operations of fresh and frozen fruits, vegetables and herbs (ffFVHs). Part 1 (outbreak data analysis, literature review and stakeholder questionnaire)

“…However, suitable/qualified surrogates for validating the performance of ffFVHs handing and processing operations are presently lacking (Gombas et al., 2017). Therefore, specific studies will be needed to qualify a potential surrogate microorganism showing a similar behaviour as compared to the target pathogen (Deng et al., 2014). Moreover, challenge testing is costly, especially as all deviations of the process must be considered to cover the variability range of relevant operating conditions (as defined in step [2]) within and between production batches/runs.…”